Poisonous

One of my friends who I met when I was in UCSD contacted me several days ago.  He emailed me to chat and catch up.  It was good to hear from him again.

He is the only long lasting friend I made during my time at UCSD.  Even though I had other friends and could have had other friends when I was at UCSD, I watched as relation after relation crash and burn.  At the time I blamed it on crappy people who tried to use me or on my social awkwardness that created misunderstandings, resulting in deteriorating relations.  But is it really?  Are these the real reasons or are they merely excuses?  What if I am poisonous to the people around me?  I always manage to screw it up resulting in me inadvertently hurting the people around me.  Every time someone gets close to me, I always end up pushing them away.  If I am poisonous, should I even try to get close to others or should I just isolate myself?  I don't know what to do.